1. Field of the Invention
The present invention relates generally to the field of fuel cells and, more specifically, to a direct methanol fuel cell system in which active control of the concentration of methanol at a critical point within the cell minimizes crossover of methanol through the cell""s membrane.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suitable for use as a fuel, depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or formaldehyde, are attractive choices for fuels due to their high specific energies.
Fuel cell systems may be divided into xe2x80x9creformer basedxe2x80x9d (i.e., those in which the fuel is processed in some fashion before it is introduced into the cell) or xe2x80x9cdirect oxidationxe2x80x9d in which the fuel is fed directly into the cell without internal processing. Most currently available fuel cell systems are of the reformer-based type and their fuel processing requirement limits their application to relatively large applications relative to a direct oxidation system.
An example of the direct oxidation system is the direct methanol fuel cell system or DMFC. In a DMFC, the electrochemical reaction at the anode is a conversion of methanol and water to CO2, H+ and exe2x88x92. The hydrogen ions flow through a membrane electrolyte to the cathode, while the free electrons flow through a load which is normally connected between the anode and cathode providing power to the load. At the cathode, oxygen reacts with hydrogen ions and free electrons to form water.
Conventional DMFCs suffer from a problem which is well known to those skilled in the art: cross-over of methanol from the anode to the cathode through the membrane electrolyte, which causes significant loss in efficiency. Cross-over occurs because of the high solubility of methanol in the membrane electrolyte. In order to minimize cross-over, and thereby minimize the loss of efficiency, the concentration of methanol in the fuel feed stream is kept low (e.g., below 1M) by dilution with water. However, dilution of the methanol introduces other disadvantages: (1) the fuel cell""s construction becomes more complicated and costly because of the structures and processes needed to store and manage the water; and (2) the energy per unit volume of the fuel cell, which is a critical factor in terms of the fuel cell""s potential commercial applications, is reduced.
In brief summary, the present invention provides a direct methanol fuel cell system in which, in response to changes in the output power level of the cell, the concentration of methanol supplied to the anode is actively controlled, thereby minimizing methanol cross-over and maintaining efficiency over a wide operating range. Mechanisms for controlling the methanol concentration are preferably constructed using microelectromechanical system (MEMS) fabrication techniques which enable the control mechansim to be readily integrated with the fuel cell""s structure.